The Blue Brick pictures were inspired by my (ex) hobby of pottery and at that
time I lived near the Aldridge Brick Company.

But as is the habbit of things the photography continues wherever something
is seen.

Blue bricks are characterised by their high iron content, fired at higher temperatures.
This can turn their surface blue/black by reduction or sometime red depending on the kind of firing.
Gas kilns take oxygen from the iron oxide making ferrous oxide, electric kilns don't. Which leaves the redder ferric oxide (ie rust).
All of which makes for a harder brick that is far less porous. They get used on canals and damp-proof courses (eg).